This invention relates to a power converter and, in particular, to a current resonance type DC/DC converter including a resonance circuit and a method of actualizing a zero-current switching therefor.
In the manner which is well known in the art, the DC/DC converter is a power converter for converting an input DC voltage (which will later be merely also called an “input voltage”) into an output DC voltage (which will later be merely also called an “output voltage”) which is different from the input DC voltage.
As one of the DC/DC converters, there is a PWM (pulse width modulation) type DC/DC converter is known in the art. The PWM type DC/DC converters have various types which are classified into a step-down type, a step-up type, a polarity reversing type, or the like. The step-down PWM type DC/DC converter comprises an energizing switch, a short-circuit switch, and an output inductor. In lieu of the short-circuit switch, a diode may be used.
However, the PWM type DC/DC converter is disadvantageous in that it has a large switching loss when the energizing switch changes from an on state to an off state or changes from an off state to an on state. As a DC/DC converter which is capable of eliminating such a switching loss, a current resonance type DC/DC converter is known, for example, in U.S. Pat. No. 5,663,635 issued by Vinciarelli et al.
Although the current resonance type DC/DC converter will later be described in conjunction with FIG. 1, the current resonance type DC/DC converter comprises a current resonance type DC/DC converting portion which includes an energizing switch being turned on and off in response to a driving control signal and a series resonance circuit. The series resonance circuit consists of a resonance inductor and a resonance capacitor. The resonance inductor has an end connected to the energizing switch. The resonance capacitor has an end connected to another end of the resonance inductor.
In the current resonance type DC/DC converter, a current flows through the resonance inductor only for a resonance duration with respect to a switching period. The current does not flow through the resonance inductor for a duration obtained by removing the resonance duration from the switching period. When an input/output voltage ratio becomes smaller, the switching period with respect to the resonance duration becomes longer. As a result, durations where the current does not flow through the resonance inductor increase, as described, for example, in U.S. Pat. No. 4,720,667 issued by Lee et al.
The current resonance type DC/DC converter has a large advantage where a zero-current switching of the energizing switch is enable by using a series resonance of the series resonance circuit consisting of the resonance inductor and the resonance capacitor, and it results in eliminating the switching loss.
Accordingly, to take the advantage of the current resonance type DC/DC converter, it is necessary to carrying out the switching of the energizing switch by precisely detecting a time instant when the current flowing through the energizing switch becomes zero.
Conventionally, as methods of actualizing the zero-current switching, which will later also be called ZCS for short, first and second conventional methods are adopted in the manner which will be later described in conjunction with FIGS. 2 and 3. The first conventional method (ZCS) is a method (ZCS) of inserting a detection resistor in series in the circuit. The second conventional method (ZCS) is a method (ZCS) of using a voltage drop due to an ON resistance of the energizing switch and a parasitic resistance of the resonance inductor.
The current resonance type DC/DC converter actualizing the first conventional ZCS comprises the detection resistor and a zero-current detection circuit. The current resonance type DC/DC converter actualizing the second conventional ZCS includes a resistance component extraction type zero-current detection circuit.
Incidentally, in practical products, the resonance frequency determined by a time constant of the resonance inductor and the resonance capacitor of the current resonance type DC/DC converting portion is several MHz or more. Therefore, in the ZCS actualizing methods, it is necessary to make a resistance value of the detection resistor or a resistance value of a combined resistance component of the energizing switch and the resonance inductor a sufficient large value in comparison with a parasitic impedance component of respective parts, patterns, and so on.
To name a concrete example thereof, it will be assumed that the resonance frequency determined by the time constant of the resonance inductor and the resonance capacitor of the current resonance type DC/DC converting portion is 2 MHz. The combined resistance component has the resistance value of several mΩ while the parasitic inductance component has an inductance value of hundreds of mH. Under the circumstances, it is impossible to precisely detect the current flowing through the energizing switch.
However, if the resistance value of the detection resistor or the resistance value of the combined resistance component makes larger than that of the parasitic inductance component, it is impractical because the current resonance type DC/DC converting portion has a large loss.
Conversely, it may be considered to detect the current flowing through the energizing switch by using, as the resistance value of the detection resistor or the resistance value of the combined resistance component, a small value which becomes negligible as the loss of the current resonance type DC/DC converting portion. In this even, however, a high-precision and complicated circuit becomes required as the zero-current detection circuit or the resistance component extraction type zero-current detection circuit.